Radiant Floor Heating Calculator

Plan a heated floor with this radiant floor heating calculator: PEX length, loop count, manifold ports, electrical load and the BTU per hour the floor can deliver.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Radiant tubing is floor area divided by the spacing in feet, plus a leader out and back per loop, with loops near 300 ft. A 300 sq ft slab at 12 in on center with a 20 ft leader takes 380 ft of PEX in 2 loops, delivering 26.4 BTU/h per sq ft.

Your measurements

Plan view from above: area is length times width.

Heated floor areasq ftOnly the area that gets tubing or mat. Deduct the footprint of a tub, a shower base, cabinets and anything else fixed to the floor, because heat under them is wasted.System typeA slab pour puts the tube in concrete or gypsum over insulation. Staple-up hangs it under the subfloor with plates and gives up roughly a third of the output. Electric is for one room, not a house.Tube or cable spacing12 in on center is the standard slab layout. Tighten to 9 or 6 in along an exterior wall or under a full glass wall, where the loss per square foot is highest.Floor coveringThis is the single biggest lever on output. Tile passes heat almost freely; carpet and pad block it. Keep the total finish assembly at R-1 or less.Room heat loss (BTU per hour per sq ft)From a Manual J or a heat loss calculation, divided by the floor area. A tight new house runs 15 to 20, an average house 25 to 30, and a sunroom or a poorly insulated addition 35 and up.Supply water temperature (F)Slabs run 100 to 120 F, staple-up 130 to 150 F because the subfloor is in the way. Above 140 F a mixing valve is required so the floor surface stays under 85 F.Leader run to the manifoldftDistance from the manifold to where the loop starts. It is counted twice per loop, out and back, and it eats into the 300 ft a loop is allowed.

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Results

Tubing needed: 380 ft; 300 sq ft at 12 in on center is 300 ft in the field, plus 40 ft of leader per loop. Buy oxygen-barrier PEX, and IRC M2103.2 wants a thermal barrier under the slab
Loops: 2 loops; About 190 ft each. Keep loops within 300 ft of 1/2 in PEX and near equal in length, or the short loop steals the flow
Manifold ports: 2 supply, 2 return; Buy the next size up if another zone is likely. Each port wants a balancing valve and a flow meter so the loops can be set to the same flow
Floor output: 26.4 BTU/h per sq ft; Against 25 BTU/h per sq ft needed, at 105 F average water over a 68 F room
Water temperature and flow: 115 F supply; 7,929 BTU/h total, which is 0.79 gpm across all loops at a 20 F loop drop
Planning estimate: Verify on site; Loop layout, boiler or heat pump sizing and the electrical work belong to a licensed installer. IRC M2103.3 wants embedded tubing held at 100 psi for 30 minutes before the pour

Radiant Floor Heating formula

Field tubing (ft) = Floor area (sq ft) / Spacing (ft)
Loops = Field tubing / (300 - 2 x Leader), rounded up   (300 ft max on 1/2 in PEX)
Total tubing (ft) = Field tubing + Loops x 2 x Leader
Output = (Supply F - 10 - 68) / (R slab + R covering + 0.5)   (BTU/h per sq ft)

Worked example

A 480 sq ft slab addition with tubing at 9 in on center, a 25 ft leader from the manifold, and engineered wood at R-0.60 on top. Supply water is 120 F and the heat loss is 22 BTU per hour per square foot. Field tubing is 480 / 0.75 = 640 ft. A loop can hold 300 - 50 = 250 ft of field tube, so 640 / 250 rounds up to 3 loops. Total tubing is 640 + 3 x 50 = 790 ft, about 263 ft per loop.

Output: the slab resistance at 9 in spacing is 0.85 x 0.866 = 0.74, plus R-0.60 for the wood and 0.5 for the floor surface, giving 1.84. Average water is 120 - 10 = 110 F, which is 42 degrees above the 68 F room. 42 / 1.84 = 22.9 BTU per hour per square foot, just over the 22 needed. Swap the wood for carpet on a thick pad at R-2.50 and the resistance jumps to 3.74, output drops to 11.2, and the floor cannot heat the room at all.

Quick reference

PEX feet and loop count by floor area and spacing, with a 20 ft leader on each loop
Floor area	12 in o.c.	9 in o.c.	6 in o.c.
100 sq ft	140 ft, 1 loop	173 ft, 1 loop	240 ft, 1 loop
150 sq ft	190 ft, 1 loop	240 ft, 1 loop	380 ft, 2 loops
200 sq ft	240 ft, 1 loop	347 ft, 2 loops	480 ft, 2 loops
300 sq ft	380 ft, 2 loops	480 ft, 2 loops	720 ft, 3 loops
400 sq ft	480 ft, 2 loops	653 ft, 3 loops	960 ft, 4 loops
600 sq ft	720 ft, 3 loops	960 ft, 4 loops	1,400 ft, 5 loops
800 sq ft	960 ft, 4 loops	1,267 ft, 5 loops	1,880 ft, 7 loops

Sizing the tubing or the mat

Tubing length is floor area divided by the spacing in feet. At 12 inches on center that is 1 foot of PEX per square foot, at 9 inches 1.33 feet, and at 6 inches 2 feet. Then add the leader, the run from the manifold to where the pattern starts, counted twice for every loop. Hold each loop near 300 feet of 1/2 inch PEX. Past that the pressure drop gets ahead of a standard circulator. The water also gives up too much heat before it turns around, so the far end runs cold. Keep loops on one manifold within about 10 percent of each other in length, because the shortest loop takes the flow.

Electric is a different animal and a different scale. Mats run 12 watts per square foot and loose cable around 15, so a 60 square foot bathroom is 720 to 900 watts. That is a warm floor under your feet, not a heating system, unless the room is small and well insulated. Mats are sold in fixed sizes and are cut only through the mesh, never through the cable. Buy the next size down and fill the odd corners with loose cable. Deduct the tub, the shower base and the vanity footprint from the area first, since heat under a cabinet does nothing.

Output, water temperature and the floor covering

A heated floor moves about 2 BTU per hour per square foot for every degree the surface sits above the room. Everything comes down to how hot that surface gets. Work it as a resistance stack: the concrete over the tube, the finish floor, and the film at the surface. A slab at 12 inch spacing under tile with 115 F supply water lands near 26 BTU per hour per square foot. Tighten the tube to 6 inches and it climbs past 35. Move the same tubing to a staple-up under a 3/4 inch subfloor and it drops by about a third. That is why staple-up runs 130 to 150 F water.

The floor covering decides more than the water temperature does. Tile and stone are about R-0.05 and barely register. Engineered wood is around R-0.60, laminate over a pad R-0.90, and carpet on a thick pad R-2.50. Keep the whole finish assembly at R-1 or less; past that you are raising water temperature to push heat through a blanket, and the surface temperature limit stops you anyway. Floor surface is generally held to 85 F for comfort and for the flooring warranty. A floor that cannot make the room at 85 F will not make it at all. That is a sign the room needs better insulation or a second heat source, not hotter water.

What this calculator leaves out

This sizes the emitter, not the plant. It does not pick a boiler, a heat pump or a water heater, and it does not size the circulator. The mixing valve, expansion tank, air separator and zone valves are outside it too. It does not draw the loop pattern, and a serpentine layout and a counterflow spiral give different temperature distributions across the same room. Under-slab and edge insulation R-values come from IRC Chapter 11 and the local energy code, not from here. On the electric side it leaves out the sensor placement, the wall box depth a floor thermostat needs, and the panel capacity for a new 240 V circuit. Permits, the pressure test before the pour, and the final electrical inspection are all set locally.

Code limits that apply

NEC 424.44(G)checked
GFCI protection on heated floors in bathrooms, kitchens and hydromassage tub areas
If you miss it: A nicked cable under tile in a wet room keeps the floor energized with no fault path to open the breaker
NEC 424.44(D)
Heating cable in a concrete floor may not exceed 16.5 watts per linear foot of cable
If you miss it: Overdriven cable cooks its own insulation inside the slab, and nothing can be replaced after the pour
NEC 424.3(B)checked
Fixed electric space heating is a continuous load, so the circuit is sized at 125 percent
If you miss it: A 15 A breaker on a 15 A mat nuisance trips on cold mornings until someone oversizes the breaker
NEC 210.8(A)
GFCI on 125 V receptacles in bathrooms, kitchens, basements, garages and outdoors
If you miss it: A missed GFCI at final inspection means opening the panel again after the floor is tiled and grouted
IRC M2103.2checked
A radiant floor needs a thermal barrier, R-value taken from IRC Chapter 11
If you miss it: Heat drains into the soil or the room below, and the floor never reaches setpoint on a design day
IRC M2103.3
Embedded tubing is tested at 100 psi hydrostatic for 30 minutes, joints inspected
If you miss it: A leak found after the pour means chasing it with a saw through finished concrete

Provision	Limit	If you miss it
NEC 424.44(G)checked	GFCI protection on heated floors in bathrooms, kitchens and hydromassage tub areas	A nicked cable under tile in a wet room keeps the floor energized with no fault path to open the breaker
NEC 424.44(D)	Heating cable in a concrete floor may not exceed 16.5 watts per linear foot of cable	Overdriven cable cooks its own insulation inside the slab, and nothing can be replaced after the pour
NEC 424.3(B)checked	Fixed electric space heating is a continuous load, so the circuit is sized at 125 percent	A 15 A breaker on a 15 A mat nuisance trips on cold mornings until someone oversizes the breaker
NEC 210.8(A)	GFCI on 125 V receptacles in bathrooms, kitchens, basements, garages and outdoors	A missed GFCI at final inspection means opening the panel again after the floor is tiled and grouted
IRC M2103.2checked	A radiant floor needs a thermal barrier, R-value taken from IRC Chapter 11	Heat drains into the soil or the room below, and the floor never reaches setpoint on a design day
IRC M2103.3	Embedded tubing is tested at 100 psi hydrostatic for 30 minutes, joints inspected	A leak found after the pour means chasing it with a saw through finished concrete

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Pouring over untested tubing. IRC M2103.3 wants 100 psi held 30 minutes with joints watched, and the pressure left on through the pour.
Skipping the under-slab insulation. IRC M2103.2 sends you to Chapter 11 for the R-value, and bare soil takes a third of the output.
Splicing a loop inside the slab. Run each loop unbroken from manifold to manifold; a buried fitting can never be serviced.
Loops of wildly different length on one manifold. The short loop takes the flow and the long one runs cold at its far end.
Cutting the heating cable to fit. Mats are cut through the mesh only, and a nicked cable scraps the entire run.
Laying carpet with a thick pad over a finished radiant floor. Past R-1 the water runs hotter and the room still falls short.

Key facts

A 1/2 inch PEX radiant loop is held to about 300 feet so the pressure drop stays inside what a residential circulator can push.
Tubing at 12 inches on center takes 1 foot of PEX per square foot of floor, 9 inch spacing takes 1.33 feet, and 6 inch spacing takes 2 feet.
A heated slab supplied at 110 to 120 F delivers roughly 25 to 35 BTU per hour per square foot under tile, enough for most rooms.
Electric floor mats draw 12 to 15 watts per square foot, which is 41 to 51 BTU per hour per square foot.
NEC 424.44(G) requires GFCI protection for electrically heated floors in bathrooms, kitchens and hydromassage bathtub locations.

Frequently asked questions

How much PEX do I need for radiant floor heat?

Divide the floor area by the spacing in feet, then add the leader run twice for each loop. A 300 square foot room at 12 inches on center with a 20 foot leader needs 380 feet of 1/2 inch PEX split into 2 loops.

How long can a radiant floor loop be?

About 300 feet on 1/2 inch PEX, and that includes the leader out and back. Longer loops push the pressure drop past a residential circulator and leave the far end of the loop cold.

Can I put carpet over radiant floor heating?

Only thin carpet with a low-profile pad. Keep the whole finish assembly at R-1 or less. Carpet on a thick pad is about R-2.50, which cuts output by more than half and leaves the room short on a cold day.

Does electric floor heat need its own circuit?

Yes. NEC 424.3(B) treats it as a continuous load, so the circuit is sized at 125 percent of the draw, and NEC 424.44(G) requires GFCI protection in bathrooms and kitchens. A 100 square foot mat at 12 watts is 1,200 watts, or 5 amps at 240 volts.

Sources and references

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